U.S. patent number 10,976,554 [Application Number 16/571,499] was granted by the patent office on 2021-04-13 for light guide plate and image display apparatus.
This patent grant is currently assigned to HITACHI-LG DATA STORAGE, INC.. The grantee listed for this patent is Hitachi-LG Data Storage, Inc.. Invention is credited to Takuma Kuno, Toshiteru Nakamura, Satoshi Ouchi, Ryuji Ukai.
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United States Patent |
10,976,554 |
Kuno , et al. |
April 13, 2021 |
Light guide plate and image display apparatus
Abstract
A light guide plate includes: first and second internal
reflective surfaces that are approximately parallel and propagate
incoming image light while totally reflecting the image light; a
partially reflective surface array that has a plurality of
partially reflective surfaces arranged in a direction of
propagating image light, the partially reflective surfaces being
inclined at a predetermined angle and partially reflecting the
image light; and a uniforming element that uniforms intensity
distribution of image light which is reflected by the partially
reflective surface array to be projected from the light guide
plate. As the uniforming element, the partially reflective surface
array is divided into a plurality of segments along a direction of
propagating image light, and the inter-surface spacing of the
partially reflective surfaces or the reflectivity of the partially
reflective surfaces is configured to vary from segment to
segment.
Inventors: |
Kuno; Takuma (Tokyo,
JP), Ukai; Ryuji (Tokyo, JP), Nakamura;
Toshiteru (Tokyo, JP), Ouchi; Satoshi (Tokyo,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Hitachi-LG Data Storage, Inc. |
Tokyo |
N/A |
JP |
|
|
Assignee: |
HITACHI-LG DATA STORAGE, INC.
(Tokyo, JP)
|
Family
ID: |
1000005485331 |
Appl.
No.: |
16/571,499 |
Filed: |
September 16, 2019 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200233217 A1 |
Jul 23, 2020 |
|
Foreign Application Priority Data
|
|
|
|
|
Jan 23, 2019 [JP] |
|
|
JP2019-009516 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
27/0172 (20130101); G02B 6/0035 (20130101); G02B
6/0023 (20130101); G02B 27/0081 (20130101); G02B
2027/0118 (20130101) |
Current International
Class: |
G02B
27/01 (20060101); G02B 27/00 (20060101); F21V
8/00 (20060101) |
Field of
Search: |
;385/36 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Blevins; Jerry M
Attorney, Agent or Firm: Mattingly & Malur, PC
Claims
What is claimed is:
1. A light guide plate for propagating and projecting incoming
image light, comprising: an incident surface which image light
enters; first and second internal reflective surfaces that are
approximately parallel to each other and propagate incoming image
light while totally reflecting the incoming image light; and a
partially reflective surface array that is placed in an interior
sandwiched between the first and second internal reflective
surfaces, and has a plurality of partially reflective surfaces
arranged therein in a direction of propagating image light, the
plurality of partially reflective surfaces being inclined at a
predetermined angle, the plurality of partially reflective surfaces
partially reflecting the image light; and a uniforming element that
uniforms intensity distribution of image light which is reflected
by the partially reflective surface array to be projected from the
light guide plate, wherein as the uniforming element, the partially
reflective surface array is divided into a plurality of segments
along the direction of propagating image light, and the uniforming
element has different optical configurations between the segments,
and wherein the reflectivity R of the plurality of partially
reflective surfaces falls within a range given by the following
expression: R.ltoreq.6/(EB+2.times.ER.times.tan(FOV/2)) where EB
(mm) is an eye box, ER (mm) is an eye relief, and FOV (degree) is a
field of view in a horizontal direction of image light to be
projected.
2. The light guide plate according to claim 1, wherein the segments
differ from each other in an inter-surface spacing L of the
plurality of partially reflective surfaces, and wherein the
plurality of partially reflective surfaces are arranged to satisfy
LAk+1<LAk, where LAk is the inter-surface spacing of partially
reflective surfaces of the plurality of partially reflective
surfaces located in a k-th segment of the segments from the
incident surface, and LAk+1 is the inter-surface spacing of
partially reflective surfaces of the plurality of partially
reflective surfaces located in a (k+1)-th segment of the segments
from the incident surface.
3. The light guide plate according to claim 2, wherein the
plurality of partially reflective surfaces are approximately equal
in reflectivity R to one another.
4. The light guide plate according to claim 1, wherein the
plurality of partially reflective surfaces are arranged to satisfy
the following expression:
L.sub.Ak.times.(1-R).sup.NAk.ltoreq.L.sub.Ak+1<L.sub.Ak where R
is reflectivity of the plurality of partially reflective surfaces,
L.sub.Ak is an inter-surface spacing of the partially reflective
surfaces located in the k-th segment from the incident surface, and
N.sub.Ak is the number of partially reflective surfaces.
5. The light guide plate according to claim 1, wherein the
plurality of partially reflective surfaces differ in reflectivity R
between the segments.
6. The light guide plate according to claim 1, wherein of the first
and second internal reflective surfaces, all or a portion of an
outer side of a surface from which image light exits is applied
with a light control coating layer, and wherein the uniforming
element varies transmittance of the light control coating layer
among the plurality of segments.
7. The light guide plate according to claim 1, wherein of the first
and second internal reflective surfaces, all or a portion of an
outer side of a surface from which image light exits is applied
with a coating layer having either predetermined reflection
characteristics or transmission characteristics.
8. The light guide plate according to claim 1, wherein at least one
of the plurality of partially reflective surfaces of the partially
reflective surface array has different reflectivity.
9. An image display apparatus displaying an image, comprising: an
image generation unit that generates image light of an image to be
displayed; a light guide plate that propagates and projects
incoming image light; and a coupling prism that emits image light
generated by the image generation unit to the light guide plate,
wherein the light guide plate includes: an incident surface which
image light enters, first and second internal reflective surfaces
that are approximately parallel to each other and propagate
incoming image light while totally reflecting the incoming image
light, and a partially reflective surface array that is placed in
an interior sandwiched between the first and second internal
reflective surfaces, and has a plurality of partially reflective
surfaces arranged therein in a direction of propagating image
light, the plurality of partially reflective surfaces being
inclined at a predetermined angle, the plurality of partially
reflective surfaces partially reflecting the image light, and a
uniforming element that uniforms intensity distribution of image
light which is reflected by the partially reflective surface array
to be projected from the light guide plate, wherein as the
uniforming element, the partially reflective surface array is
divided into a plurality of segments along the direction of
propagating image light, and the uniforming element has different
optical configurations between the segments, and wherein the
reflectivity R of the plurality of partially reflective surfaces
falls within a range given by the following expression:
R.ltoreq.6/(EB+2.times.ER.times.tan(FOV/2)) where EB (mm) is an eye
box, ER (mm) is an eye relief, and FOV (degree) is a field of view
in a horizontal direction of image light to be projected.
10. A head-mounted display worn on a head of a user to display an
image to the user, comprising: an image display apparatus that
displaying an image; a sensing section that detects external
information; a communication section that communicates with
external equipment; a power supply section that supplies electric
power; a storage section that stores information; an operation
input section that receives operation of the user; and a control
section that controls the head-mounted display, wherein the image
display apparatus includes an image generation unit that generates
image light of an image to be displayed, a light guide plate that
propagates and projects incoming image light, and a coupling prism
that emits image light generated by the image generation unit to
the light guide plate, and wherein the light guide plate includes:
an incident surface which image light enters, first and second
internal reflective surfaces that are approximately parallel to
each other and propagate incoming image light while totally
reflecting the incoming image light, and a partially reflective
surface array that is placed in an interior sandwiched between the
first and second internal reflective surfaces, and has a plurality
of partially reflective surfaces arranged therein in a direction of
propagating image light, the plurality of partially reflective
surfaces being inclined at a predetermined angle, the plurality of
partially reflective surfaces partially reflecting the image light,
and a uniforming element that uniforms intensity distribution of
image light which is reflected by the partially reflective surface
array to be projected from the light guide plate, wherein as the
uniforming element, the partially reflective surface array is
divided into a plurality of segments along the direction of
propagating image light, and the uniforming element has different
optical configurations between the segments, and wherein the
reflectivity R of the plurality of partially reflective surfaces
falls within a range given by the following expression:
R.ltoreq.6/(EB+2.times.ER.times.tan(FOV/2)) where EB (mm) is an eye
box, ER (mm) is an eye relief, and FOV (degree) is a field of view
in a horizontal direction of image light to be projected.
Description
CLAIM OF PRIORITY
The present application claims priority from Japanese patent
application serial No. JP 2019-009516, filed on Jan. 23, 2019, the
content of which is hereby incorporated by reference into this
application.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a light guide plate that
propagates and projects incoming image light, and an image display
apparatus.
(2) Description of the Related Art
A light guide plate used in image display apparatus, such as a
head-mounted display, a head-up display and the like, has the
function of extending an eye box through expansion and projecting
an image to a user. As technology of pupil expansion, for example,
Japanese Unexamined Patent Application Publication (Translation of
PCT Application) No. 2003-536102 discloses "an optical device
including a light-transmitting substrate, optical means for
coupling light into the substrate by total internal reflection, and
a plurality of partially reflective surfaces carried by the
substrate, in which the partially reflective surfaces are parallel
to each other and are not parallel to any of the edges of the
substrate".
SUMMARY OF THE INVENTION
There is a need for the light guide plate to have high see-through
characteristics to prevent a user's view from being blocked, in
addition to extend the eye box to a predetermined size by pupil
expansion. In particular, the head-mounted displays are designed in
mind to be used in the aid in daily life, in work support such as
in maintenance and checkups and the like. Accordingly, the light
guide plate and the entire image display apparatus including the
light guide plate are required to have high light use efficiency in
order to provide bright display image.
In the configuration disclosed in Japanese Unexamined Patent
Application Publication (Translation of PCT Application) No.
2003-536102, the partially reflective surfaces are placed as a
light guide plate (optical device) in the interior of the
transparent substrate in order to project image light toward the
user. However, while the image light propagates through the light
guide plate and is projected by the partially reflective surfaces,
the amount of image light propagating through the light guide plate
gradually decreases. This makes it impossible to project the image
light of uniform brightness. In addition, if the reflectivity of
the partially reflective surfaces is increased to achieve high
light use efficiency, the nonuniformity of brightness of the image
right will likely become increasingly salient. Thus, it is
difficult to project image light of uniform brightness in the light
guide plate with maintenance of high light use efficiency.
The present invention has been achieved in view of such problems,
and it is an object to provide a light guide plate capable of
projecting image light of uniform brightness with high light use
efficiency, and an image display apparatus.
To achieve the object, a light guide plate according to an aspect
of the present invention includes: an incident surface which image
light enters; first and second internal reflective surfaces that
are approximately parallel to each other and propagate incoming
image light while totally reflecting the incoming image light; a
partially reflective surface array that is placed in an interior
sandwiched between the first and second internal reflective
surfaces, and has a plurality of partially reflective surfaces
arranged therein in a direction of propagating image light, the
plurality of partially reflective surfaces being inclined at a
predetermined angle and partially reflecting the image light; and a
uniforming element that uniforms intensity distribution of image
light which is reflected by the partially reflective surface array
to be projected from the light guide plate.
Specifically, as the uniforming element, the partially reflective
surface array is divided into a plurality of segments along the
direction of propagating image light, and the uniforming element
has different optical configurations between the segments. For
example, an inter-surface spacing L of the partially reflective
surfaces or the reflectivity R of the partially reflective surfaces
is configured to vary from segment to segment.
According to the present invention, a light guide plate capable of
projecting image light of uniform brightness with high light use
efficiency and an image display apparatus can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, objects and advantages of the present
invention will become more apparent from the following description
when taken in conjunction with the accompanying drawings
wherein:
FIG. 1 is a diagram illustrating the configuration of an image
display apparatus 1 according to a first embodiment;
FIGS. 2A and 2B are diagrams illustrating an example configuration
of a conventional light guide plate 2';
FIG. 3 is a diagram illustrating the positional relationship
between a user's pupils and a light guide plate;
FIGS. 4A and 4B are diagrams illustrating the configuration of a
light guide plate 2a according to the first embodiment;
FIG. 5 is an explanatory diagram of the inter-surface spacing
condition with consideration of a thickness of the light guide
plate 2a;
FIGS. 6A, 6B and 6C are diagrams showing the results of the
simulation of image-light intensity distribution;
FIGS. 7A and 7B are diagrams illustrating example applications of
the image display apparatus 1;
FIGS. 8A and 8B are diagrams illustrating a user 50 wearing a head
mounted display 20;
FIG. 9 is an enlarged diagram of the light guide plate 2a placed in
an inclined position;
FIG. 10 is a block diagram illustrating the functional
configuration of the head mounted display 20;
FIGS. 11A and 11B are diagrams illustrating the configuration of a
light guide plate 2b according to a second embodiment;
FIGS. 12A and 12B are diagrams illustrating the configuration of a
light guide plate 2c according to a third embodiment;
FIGS. 13A and 13B are diagrams illustrating the configuration of a
light guide plate 2d according to a fourth embodiment;
FIGS. 14A and 14B are diagrams illustrating the configuration of a
light guide plate 2e according to a fifth embodiment; and
FIGS. 15A and 15B are diagrams illustrating the configuration of a
light guide plate 2f according to a sixth embodiment.
DETAILED DESCRIPTION OF THE EMBODIMENT
Some of embodiments according to the present invention will now be
described with reference to the accompanying drawings. Each
embodiment has a uniforming element that uniforms brightness of
image light projected from a light guide plate. Throughout the
figures, like reference signs refer to elements having the same or
similar functions, but different reference signs 2a to 2f are used
to designate a light guide plate 2 for distinctions among first to
sixth embodiments.
First Embodiment
In a first embodiment, the uniforming element is configured to set
different spacings between the partially reflective surfaces within
the light guide plate.
FIG. 1 is a schematic diagram illustrating the configuration of an
image display apparatus 1 according to the first embodiment, in
which a user watches an image from the image display apparatus 1.
Assuming that the left-right direction with respect to the user
(user's pupil 51) is defined as the x axis, the front-rear
direction (line-of-sight direction) is defined as the y axis and
the up-down direction is defined as the z axis, and here an x-y
sectional view is shown when seen from above the user's head. The
image display apparatus 1 includes an image generation unit 3, a
coupling prism 4, and a light guide plate 2a which is placed along
the x axis.
The image generation unit 3 generates an image to be watched by the
user, and then emits the image light to the coupling prism 4. The
coupling prism 4 couples the image generation unit 3 and the light
guide plate 2a to each other in order to direct the image light
emitted from the image generation unit 3, toward the light guide
plate 2a. Specifically, the coupling prism 4 has a first surface 41
and a second surface 42 which form a vertex angle .alpha.. The
image light enters from the image generation unit 3 to the fist
surface 41 and then exits from the second surface 42 to be incident
to an incident surface 5 of the light guide plate 2a.
The light guide plate 2a propagates in the x direction the image
light entering through the incident surface 5, and uses a plurality
of partially reflective surfaces 8 to project the image light in
the y direction toward the user's pupil 51 so that the user can
visually recognize the image. At this time, an exit pupil is
configured to be expanded in the x direction in order to expand an
eye box (the range where the user can visually recognize the
image). It is noted that the coupling prism 4 may be configured to
have the function of enlarging the exit pupil in the z direction to
expand the eye box in the z direction.
The light guide plate 2a includes a first internal reflective
surface 6 and a second internal reflective surface 7 which are
approximately parallel to each other. The light guide plate 2a has
a partially reflective surface array 10 in the interior sandwiched
between the first internal reflective surface 6 and the second
internal reflective surface 7. The partially reflective surface
array 10 has the plurality of partially reflective surfaces 8 which
are arranged at an inclination angle .theta. in the x direction.
The image light entering from the coupling prism 4 through the
incident surface 5 is totally reflected by the first internal
reflective surface 6 and the second internal reflective surface 7
to propagate in the x direction. Also, a portion of the image light
propagating in the x direction is reflected off the plurality of
partially reflective surfaces 8 to be redirected in the y
direction, which then passes through the first internal reflective
surface 6 to exit to the outside of the light guide plate 2a. At
this time, the image light reflected off the plurality of partially
reflective surfaces 8 is replicated, so that the eye box is
expanded in the x direction.
A portion of the image light exiting to the outside of the light
guide plate 2a enters the user's pupil 51. Thus, the user can
visually recognize the image displayed by the image display
apparatus 1. The higher the reflectivity of the partially
reflective surfaces 8, the greater the amount of image light
projected to the user, resulting in a light guide plate with high
light use efficiency.
An end 9 of the light guide plate 2a is formed to be approximately
parallel to the incident surface 5, and prevented from being
orthogonal to the first and second internal reflective surfaces 6,
7. The light guide plate end 9 has preferably a polished surface
rather than a sand surface (ground glass surface). Thereby, the
image light, which has been totally reflected to propagate in the
interior of the light guide plate 2a and then has passed through
all the partially reflective surfaces 8, becomes apt to pass
through the light guide plate end 9 to make stray light
unlikely.
It is noted that a light shield unit 11 may be installed on the
outside of the light guide plate end 9 in order to block the light
passing through the light guide plate end 9. The light shield unit
11 includes a light shield wall, a light shield block, a light
shield sheet and/or the like, and is capable of reducing the stray
light visually recognized by the user.
In the light guide plate 2a according to the first embodiment, an
inter-surface spacing L of the plurality of partially reflective
surfaces 8 arranged in the partially reflective surfaces array 10
is varied in the arrangement direction, whereby the high light use
efficiency can be maintained as well as the light quantity
distribution of the projected image light can be uniformed.
<Configuration and Performance of Conventional Light Guide Plate
2'>
A configuration of conventional light guide plates and issues
involved therein are discussed for the purpose of comparison.
FIGS. 2A and 2B are diagrams illustrating an example configuration
of a conventional light guide plate 2', in which FIG. 2A shows a
front view when seen from the user, and FIG. 2B shows a plan view
(x-y sectional view).
The light guide plate 2' includes the first internal reflective
surface 6 and the second internal reflective surface 7 which are
approximately parallel to each other. The light guide plate 2' has
the partially reflective surface array 10 including the plurality
of partially reflective surfaces 8 arranged therein. The
conventional light guide plate 2' has an equal inter-surface
spacing L for all the partially reflective surfaces 8, and all the
partially reflective surfaces 8 are also equal in reflectivity. In
this configuration, there is an issue on nonuniform light quantity
distribution of the projected image light.
The number N of partially reflective surfaces 8 are individually
denoted as 8.sub.n in order from the incident surface 5 (n is an
integer from 1 to N). It is assumed that reflectivity of each
partially reflective surface 8 is R (where 0<R<1) and that an
inter-surface spacing in a direction perpendicular to the
reflecting surfaces of adjacent partially reflective surfaces 8 is
L. It is also assumed that spacing between adjacent partially
reflective surfaces 8 on the first internal reflective surface 6
(or the second internal reflective surface 7) is width H. Further
it is assumed that intensity of light entering through the incident
surface 5 of the light guide plate is I.sub.0, that intensity of
light reflected off the partially reflective surface 8.sub.n is
I.sub.n, and that intensity of light per unit length between light
beams I.sub.n and I.sub.n+1 is U.sub.n.
The following relationship is between the inter-surface spacing L
and width H of the partially reflective surfaces 8 when the angle
formed by the partially reflective surface 8 and the second
internal reflective surface 7 is assumed as .theta.. L=H.times.sin
.theta. (1)
Also, using the reflectivity R of the partially reflective surface
8, I.sub.n is represented as:
I.sub.n=(1-R).sup.n-1.times.R.times.I.sub.0 (2)
The light intensity U.sub.1 per unit length between light beams
I.sub.1 and 1.sub.2 is equal to a value obtained by dividing a mean
value of light intensity of I.sub.1 and I.sub.2 by width H.
U.sub.1=(I.sub.1+I.sub.2)/2/H (3)
Likewise, the light intensity U.sub.n per unit length between light
beams I.sub.n and 1.sub.n+1 is represented as:
U.sub.n=(I.sub.n+I.sub.n+1)/2/H (4)
Next, available reflectivity of the partially reflective surfaces 8
of the conventional light guide plate 2' is determined. When the
degree of uniformity K of the image light projected by the light
guide plate is defined by use of a ratio between the light
intensity U.sub.1 per unit length in a position closet to the
incident surface 5 and the light intensity U.sub.N-1 per unit
length in a position farthest from the incident surface 5, i.e.,
U.sub.N-1/U.sub.1, it is represented as:
K=U.sub.N-1/U.sub.1=(1-R).sup.N-2 (5)
In this manner, as to the number N of partially reflective surfaces
8, the larger the N is, the lower the degree of uniformity K is. As
to the reflectivity R, there is a tradeoff relationship in which
the higher the reflectivity R is, the higher the light use
efficiency is achieved, but the degree of uniformity decreases. The
degree of uniformity K of 0.5 or greater, more preferably, 0.7 or
greater, is required for the user to recognize visually an image of
uniform brightness.
Here, the N partially reflective surfaces 8 and the reflectivity R
are connected with the visibility width viewed by the user.
FIG. 3 is a diagram illustrating the positional relationship
between the user's pupils and the light guide plate 2', and using
the positional relationship, a required number of partially
reflective surfaces 8 is determined. It is assumed that the
distance from the midpoint of the partially reflective surface 81
closest to the incident surface 5 to the midpoint of the partially
reflective surface 8.sub.N farthest from the incident surface 5 is
S. Using the width H and the N partially reflective surfaces, the
distance S has the following relationship: S=H.times.(N-1) (6)
Further, it is assumed that the distance from the user's pupils 51
to the first internal reflective surface 6 (eye relief) is ER, the
eye box in the x direction is EB, and the full angle of field of
view in the x direction of the displayed image is FOV (Field of
View). From them, the length in the x direction of the existence
region of the partially reflective surfaces 8 minimally required to
display images is determined to be W (hereinafter referred to as a
"visibility width W"). In order to allow the user to recognize
visually the entire image, the distance S should have a length
equal to or greater than the visibility width W. Thus, the
following relationship must be established:
S.gtoreq.W=EB+2.times.ER.times.tan(FOV/2) (7)
As a concrete example, consider where the light guide plate has a
thickness T=1.7 mm, the partially reflective surfaces are spaced at
spacing H=2 mm, the eye relief ER=20 mm, and the eye box EB=10 mm.
When the minimum required N number of partially reflective surfaces
is determined from expressions (6) and (7), in the case of 20
degrees.ltoreq.FOV<30 degrees, N is equal to 10 partially
reflective surfaces; in the case of 30 degrees.ltoreq.FOV<40
degrees, N is equal to 12; and in the case of 40
degrees.ltoreq.FOV<50 degrees, N is equal to 14.
Further, a condition of reflectivity R for achievement of a
desirable degree of uniformity K.gtoreq.0.7 to allow the user to
recognize visually an image of uniform brightness is determined
from the minimum required N number of partially reflective surfaces
and expression (5). As a result, reflectivity R.ltoreq.5% is
required where 20 degree.ltoreq.FOV<30 degree; reflectivity
R.ltoreq.4% is required where 30 degree.ltoreq.FOV<40 degree;
and reflectivity R.ltoreq.3% is required where 40
degree.ltoreq.FOV<50 degree. If the reflectivity R of the
partially reflective surface exceeds the value, an image of uniform
brightness cannot be provided.
FIGS. 6A to 6C are diagrams showing the results of the simulations
of image-light intensity distribution. FIG. 6A shows the case
where, in the conventional light guide plate 2', when the
reflectivity R of the partially reflective surface is equal to 10%
(a value higher than the above upper limit), image light with
diagonal FOV=40 degrees is projected. FIG. 6C shows the intensity
distribution along the x axis in the vertical central position. It
is shown that, in the conventional light guide plate 2', the image
light is lower gradually toward the right of the display area, and
the brightness distribution of the displayed image becomes
non-uniform. In this way, in the conventional light guide plate 2',
the reflectivity of the partially reflective surface is required to
be set at a lower value in order to project an image light of
uniform brightness, leading to impossibility of achieving high
light use efficiency.
<Configuration and Performance of Light Guide Plate 2a in
Embodiment>
FIGS. 4A and 4B are diagrams illustrating the configuration of the
light guide plate 2a according to the first embodiment illustrated
in FIG. 1, in which FIG. 4A is a front view and FIG. 4B is a plan
view. The light guide plate 2a according to the embodiment differs
from the conventional light guide plate 2' in FIG. 2 in that the
inter-surface spacings L of the N partially reflective surfaces 8
are narrower from the light-guide-plate incident surface 5 toward
the light guide plate end 9. It is noted that the N partially
reflective surfaces 8 are approximately equal in reflectivity R.
The amount of image light totally reflected to propagate in the
interior of the light guide plate 2a decreases as the image light
travels from the incident surface 5 toward the light guide plate
end 9 because of partial reflection on the partially reflective
surfaces 8. In compensation for this, the inter-surface spacings L
of the partially reflective surfaces 8 are narrower from the
incident surface 5 toward the light guide plate end 9 in order to
increase the luminous flux density of the image light for
brightness uniformization of the image light to be projected. As a
result, although the light guide plate 2a adopts the partially
reflective surfaces 8 having approximately equal reflectivity, the
projection of image light of uniform brightness is enabled.
The following description is about the inter-surface spacing L and
the reflectivity R of partially reflective surfaces.
<About Inter-surface Spacing L of Partially Reflective Surfaces
8>
The spacing between the n-th partially reflective surface 8.sub.n
and the (n+1)-th partially reflective surface 8.sub.n+1 is denoted
as L.sub.n (n is an integer from 1 to N-1). Also, the spacing
between the n-th partially reflective surface and the (n+1)-th
partially reflective surface on the first internal reflective
surface 6 (or the second internal reflective surface 7) is denoted
as width H.sub.n.
To display an image of uniform brightness, a desirable
inter-surface spacing of the partially reflective surfaces 8 is
described. It is assumed that intensity of light entering through
the incident surface 5 of the light guide plate is I.sub.0, that
intensity of light reflected off the partially reflective surface
8.sub.n is I.sub.n, that and intensity of light per unit length
between light beams I.sub.n and I.sub.n+1 is U.sub.n. The width
H.sub.n and the inter-surface spacing L.sub.n of the partially
reflective surfaces 8.sub.n have the following relationship:
L.sub.n=H.sub.n.times.sin .theta. (8)
Also, using the reflectivity R of the partially reflective surface
8, I.sub.n is represented as:
I.sub.n=(1-R).sup.n-1.times.R.times.I.sub.0 (9)
Assuming that intensity of light per unit length between light
beams I.sub.1 and 1.sub.2 is U.sub.1, the light intensity U.sub.1
can be expressed as a value obtained by dividing a mean value of
light intensity of I.sub.1 and I.sub.2 by width H.sub.1.
U.sub.1=(I.sub.1+I.sub.2)/2/H.sub.1 (10)
Likewise, the light intensity U.sub.n per unit length between light
beams I.sub.n and 1.sub.n+1 is represented as:
U.sub.n=(I.sub.n+I.sub.n+1)/2/H.sub.n (11)
The relationship between adjacent inter-surface spacings L.sub.n
and L.sub.n+1 is determined. The larger n (the closer to the light
guide plate end 9 from the incident surface 5), the smaller the
light intensity I.sub.n. Because of this, in order to uniform the
image light to be projected, the inter-surface spacing L.sub.n is
decreased as n increases. Thus, the relationship
L.sub.n+1<L.sub.n is established.
Next, the lower limit of L.sub.n+1 is determined. The image light
to be projected is uniformed by equalizing adjacent light
intensities U.sub.n and U.sub.n+1 per unit length. If the
relationship between the inter-surface spacings L.sub.n and
L.sub.n+1 is determined from expressions (8) to (11),
L.sub.n+1=(1-R)L.sub.n is obtained. In this case, if L.sub.n+1 is
lower than (1-R)L.sub.n, the too small inter-surface spacing will
cause an increase in number of the partially reflective surfaces,
leading to increases in manufacturing cost. Therefore, the
relationship between the inter-surface spacings L.sub.n and
L.sub.n+1 is defined within the following range:
(1-R)L.sub.n.ltoreq.L.sub.n+1<L.sub.n (12)
TO allow the user to recognize visually an image of uniform
brightness, the degree of uniformity is required to be
K.gtoreq.0.5. Therefore, when the number of partially reflective
surfaces is N, the inter-surface spacing L.sub.n+1 is required to
fall within the following range:
(1-R)L.sub.n.ltoreq.L.sub.n+1<(1-R)L.sub.n/0.5.sup.(1/(N-2))
(13) Further, to obtain a more desirable degree of uniformity
K.gtoreq.0.7, the inter-surface spacing L.sub.n+1 is required to
fall within the following range:
(1-R)L.sub.n.ltoreq.L.sub.n+1<(1-R)L.sub.n/0.7.sup.(1/(N-2))
(14)
FIG. 5 is an explanatory diagram of the inter-surface spacing
condition with consideration of the thickness of the light guide
plate 2a. If the inter-surface spacing L between adjacent partially
reflective surfaces 8 is large, no projected light exists in a
region A to give rise to a partial loss of the image. In order to
prevent a partial loss of the image, what is required is only that,
when the user views the light guide plate from the front, the
partially reflective surfaces 8 are configured to look overlapping
one another. For this purpose, assuming that the thickness of the
light guide plate is T and the angle formed by the partially
reflective surface 8 and the second internal reflective surface 7
is .theta., the largest inter-surface spacing L.sub.1 between the
first partially reflective surface 81 and the second partially
reflective surface 82 desirably satisfies the following:
L.sub.1.ltoreq.T.times.cos .theta. (15)
Also, the too small inter-surface spacing of partially reflective
surfaces 8 causes an increase in number of the partially reflective
surfaces 8. Therefore, in terms of the lowering of costs, L.sub.1
desirably satisfies the following: L.sub.1.ltoreq.T.times.cos
.theta./2 (16) The above is the condition of the inter-surface
spacing L of the partially reflective surfaces 8 for displaying an
image of uniform brightness.
<About Reflectivity of Partially Reflective Surface 8>
The following is a description of the condition of the reflectivity
of the partially reflective surface 8. The reflectivity R of the
partially reflective surface 8 has a tradeoff relationship in which
because the amount of light projected to the user increases as
reflectivity is higher, the light use efficiency of the light guide
plate is increased, but the see-through characteristics is
degraded. Typically, the reflectivity of 30% or lower is desirable
in terms of the see-through characteristics.
The reflectivity R of the partially reflective surface 8 preferably
exhibits a lower independence of wavelength in the wavelength
region of visible light. For example, if a ratio of maximum
reflectivity to minimum reflectivity in the visible light region is
50% or higher, the user does not perceive much of the nonuniformity
between the image and the outside world. And, if the ratio is 80%
or higher, the user becomes hardly perceive the nonuniformity
between the image and the outside world. As a result, the color
uniformity of the outside world and the color uniformity of the
image perceived by the user can be ensured.
It is noted that the partially reflective surface 8 may be realized
by use of a metallic film or use of a dielectric multilayer film.
Alternatively, a polarizing beam splitter may be used which relies
on the polarization of image light to split the light.
The following is a description of the range of reflectivity of the
partially reflective surface 8. As described in FIG. 3, it is
assumed that the length in the x direction of the region of the
light guide plate in which the partially reflective surfaces are
placed is S. It is also assumed that the distance from the user's
pupils 51 to the first internal reflective surface 6 (eye relief)
is ER, that the eye box in the x direction is EB, and that the full
angle of field of view in the x direction of the displayed image is
FOV (Field of View). In order to allow the user to recognize
visually the entire image, the distance S should have a length
equal to or greater than the visibility width W as shown in FIG. 3.
Thus, the following relationship must be established:
S.gtoreq.W=EB+2.times.ER.times.tan(FOV/2) (17)
There is a lower limit of manufacturable inter-surface spacing L of
the partially reflective surfaces in FIG. 4B. Where the lower limit
of the inter-surface spacing is a minimum inter-surface spacing a,
the minimum inter-surface spacing is typically the order of 0.2 mm.
Even in high reflectivity, the uniformization can be achieved by
decreasing the inter-surface spacing. However, as described above,
there is a lower limit of manufacturable inter-surface spacing of
the partially reflective surfaces, and therefore there is a limit
on available reflectivity in the minimum inter-surface spacing
a.
Consider the instance where the partially reflective surfaces are
arranged in L.sub.n+=(1-R)).times.L.sub.n, such that the image
light to be projected is most uniformed. At this time, the smallest
inter-surface spacing is L.sub.N-1 farthest away from the incident
surface 5, which is expressed by: L.sub.N-1=L.sub.1(1-R)).sup.(N-2)
(18) At this time width H.sub.N-1 is expressed by:
H.sub.N-1L.sub.N-1/sin .theta.=H.sub.1(1-R)).sup.(N-2) (19) In
terms of manufacturing, L.sub.N-1 should be equal to or greater
than the minimum inter-surface spacing a, so that the following is
required: a.ltoreq.L.sub.N-1=L.sub.1(1-R).sup.(N-2) (20)
Assuming A=a/sin .theta., Expression (20) is expressed under the
condition of width H.sub.N-1, as follows: A=a/sin
.theta..ltoreq.H.sub.N-1=H.sub.1(1-R).sup.(N-2) (21) Also, distance
S can be expressed as follows: S=H.sub.1+H.sub.2+ . . .
+H.sub.N-1=H.sub.1/R.times.(1-(1-R).sup.(N-1) (22) From expressions
(21) and (22), the condition of available reflectivity R can be
determined as: R.ltoreq.(H.sub.1-A)/(S-A) (23)
The physical sense of expression (23) is considered. If there is no
lower limit of manufacturable inter-surface spacing of the
partially reflective surfaces, when A=0, an upper limit of
reflectivity is H.sub.1/S. Thus, the upper limit of available
reflectivity depends on a proportion of the width H.sub.1 of a
single partially reflective surface to the overall width S in which
the partially reflective surfaces are arranged. If there is a lower
limit of the manufacturable inter-surface spacing of the partially
reflective surfaces, the larger the minimum inter-surface spacing a
is, that is, the larger A, the lower the upper limit of available
reflectivity is.
Also, using H.sub.1=L.sub.1/sin .theta. and expressions (17) and
(21), expression (23) takes the form of: R.ltoreq.(T cos
.theta.-a)/(W sin .theta.-a) (24) As described above, "a" is the
minimum inter-surface spacing, T is the thickness of the light
guide plate, and angle .theta. is an angle formed by the partially
reflective surface 8 and the second internal reflective surface 7.
Using the eye relief ER, the eye box EB, and the full angle of
field of view FOV in the x direction of the displayed image, the
visibility width W can be written as
W=EB+2.times.ER.times.tan(FOV/2).
In order to place the light guide plate in front of the user's
eyes, in terms of designability, the thickness T is preferably as
thin as possible, and thickness T.ltoreq.3 mm is required. Also,
the minimum inter-surface spacing is the order of a=0.2 mm, and
when angle .theta.=25 degrees, when expression (24) is written
under the above limit conditions, using eye relief ER and eye box
EB and FOV, R.ltoreq.6/(EB+2.times.ER.times.tan(FOV/2)) (25) is
obtained. It is noted that "a" in the denominator of expression
(24) is neglected because it is small as compared with W sin
.theta.. As shown in expression (25), the larger the eye relief ER,
eye box EB and FOV are, the lower the upper limit of available
reflectivity of the partially reflective surface is.
When the eye relief ER=20 mm, and the eye box EB=10 mm, if a
specific limit of the reflectivity R is determined from expression
(25), in the case of 20 degrees .ltoreq.FOV<30 degrees, the
reflectivity R may be set at 30% or lower; in the case of 30
degrees.ltoreq.FOV<40 degrees, the reflectivity R may be set at
25% or lower; and in the case of 40 degrees.ltoreq.FOV<50
degrees, the reflectivity R may be set at 21% or lower.
With the light guide plate 2a according to the embodiment, the
range of available reflectivity is extended as compared with the
conventional art. For example, in the use in 30
degrees.ltoreq.FOV<40 degrees, the conventional light guide
plate 2' requires reflectivity R.ltoreq.4%, whereas the range of
reflectivity R in the light guide plate in the embodiment is
extended to be equal to or less than 25%. As a result, the light
use efficiency of the light guide plate improves about 2.9-fold as
compared with the conventional light guide plate.
FIG. 6B shows the image-light intensity distribution by the light
guide plate 2a according to the embodiment. The simulation
conditions of image projection are that: the partially reflective
surfaces are arranged such that the spacing of the partially
reflective surfaces is expressed using an equal sign in expression
(12), i.e., L.sub.n+1=(1-R)L.sub.n is achieved; the reflectivity R
of the partially reflective surface is equal to 10%; and the
diagonal FOV is equal to 40 degrees. FIG. 6C also shows the
intensity distribution along the x axis. As compared with the
distribution of the conventional light guide plate shown in FIG.
6A, it is seen that the light guide plate according to the
embodiment shown in FIG. 6B is improved in intensity distribution,
and the brightness distribution of the displayed image can be
uniformed.
According to the embodiment as described above, the inter-surface
spacings of the partially reflective surfaces are narrower from the
incident surface toward the light guide plate end. Thereby, even
when high reflectivity is set for the partially reflective
surfaces, the projection of image light of uniform brightness is
enabled, thus providing a light guide plate capable of projecting
image light of uniform brightness with high light use
efficiency.
It is noted that applying a coating having chemical properties to
the exit surface of the light guide plate can increase the
performance of the light guide plate. For example, if an
Anti-reflection (AR) coating layer is applied to the outer side of
the first internal reflective surface 6, the light use efficiency
can be increased and also unwanted stray light can be eliminated.
Additionally or alternatively, by applying a light control coating
layer having transmittance distribution to the outer side of the
first internal reflective surface 6, the intensity distribution of
the image light can be further uniformed.
Further, in the above description, the partially reflective
surfaces are approximately equal in reflectivity. However, no
problem is presented even if the partially reflective surfaces are
not equal in reflectivity. For example, the intensity distribution
of image light can be further uniformed by making the inter-surface
spacings of the partially reflective surfaces different from each
other, and also by adjusting the reflectivity of the partially
reflective surfaces for each reflective surface.
The following is a description of components within the image
display apparatus 1 except the light guide plate 2a.
<About Image Generation Unit 3>
The image generation unit 3 in FIG. 1 includes a light source, an
illumination optics, an image generation element generating an
image, and a projection optics for projecting image light.
Examples of the light source include an RGB LED, and an RGB LD. As
a matter of course, a white LED may be used as a light source. In
this case, there is a need for the image generation element to be
equipped with a color filter. The illumination optics illuminates
uniformly the image generation element with light of the light
source. For the image generation element, a liquid crystal device,
a digital mirror device (DMD) or the like may be used. The
projection optics includes a projection lens including a single
lens or multiple lenses, and projects the image light generated by
the image generation element. It is noted that, as an image
generation element, a self-luminous image generation element such
as an organic EL, a .mu.LED, or the like may be used. In this case,
the light source and the illumination optics become unnecessary,
and a reduction in size and weight of the image generation unit is
enabled.
<About Coupling Prism 4>
The coupling prism 4 in FIG. 1 is formed and placed such that the
field of view of the image light generated by the image generation
unit 3 and the field of view of the image displayed by the image
display apparatus 1 are approximately identical. For example, the
coupling prism 4 is formed and placed such that the light which is
vertically incident onto the first surface 41 of the coupling prism
4 exits approximately vertically from the first internal reflective
surface 6. Specifically, for example, it is assumed that the first
surface 41 through which the light from the image generation unit 3
enters the coupling prism 4 forms approximately an angle 2 .theta.
with the second internal reflective surface 7. Stated another way,
the coupling prism 4 is formed and placed such that the angle
formed by the first surface 41 and the second internal reflective
surface 7 is approximately twice the angle .theta. formed by the
partially reflective surface 8 and the second internal reflective
surface 7. More specifically, the coupling prism 4 is formed and
placed such that the vertex angle .alpha. of the coupling prism 4
is approximately equal to the angle .theta., and the second surface
42 through which the image light exits from the coupling prism 4,
and the incident surface 5 are approximately parallel to each
other.
Also, the coupling prism 4 and the light guide plate 2a may be
formed of the same medium or media approximately equal in
refractive index. Thereby, the entry angle of the light entering
the coupling prism 4 and the exit angle of the light exiting from
the first internal reflective surface 6 can be made approximately
equal to each other. Therefore, the field of view of the image
light generated by the image generation unit 3 and the field of
view of the image displayed by the image display apparatus 1 can be
made approximately identical with each other.
Further, the image generation unit 3 and the coupling prism 4 are
arranged such that the principal light beam at the center of the
field of view of the image light emitted from the image generation
unit 3 enters the coupling prism 4 approximately vertically.
Thereby, the principal light beam at the center of the field of
view of the image light emitted from the image generation unit 3
can exits approximately vertically from the first internal
reflective surface 6.
It is noted that the principal light beam at the center of the
field of view of the image light emitted from the image generation
unit 3 may be configured to exit, at an angle other than the right
angle, from the first internal reflective surface 6 of the light
guide plate 2a. This can be achieved by configuring, for example,
to allow the principal light beam at the center of the field of
view of the image light emitted from the image generation unit 3 to
enter, at an angle other than the right angle, the coupling prism
4.
Also, the coupling prism 4 may have a vertex angle .alpha.
different from the angle .theta.. In this case, by the effect of
beam compression or beam expansion, the field of view of the image
light emitted from the image generation unit 3 and the field of
view of the image displayed by the image display apparatus 1 can be
made different from each other.
The function of enlarging the pupil in the z direction may be
imparted to the coupling prism 4. By doing so, the eye box can be
expanded in the z direction.
Further, the coupling prism 4 may be omitted, and the outgoing
light from the image generation unit 3 may be input directly to the
incident surface 5 of the light guide plate 2a. Thereby, a
reduction in component count of the image display apparatus 1 is
enabled, and in turn a cost reduction, a mass reduction and a size
reduction are enabled.
<Example Application of Image Display Apparatus 1>
FIGS. 7A and 7B are diagrams illustrating example applications of
the image display apparatus 1. FIG. 7A shows an example of applying
the image display apparatus 1 to a head-mounted display 20. The
head-mounted display 20 is worn, for example, on the head of a user
50, and the light guide plate 2a (FIG. 1) is placed around a line
of sight of the user 50. Also, the incident surface 5 (FIG. 1) of
the light guide plate 2a may be placed in the lateral direction of
the eye of the user 50 or placed in the vertical direction of the
user's pupil 51. The user 50 can visually recognize the image
displayed by the head-mounted display 20 as, for example, a virtual
image.
FIG. 7B shows an example of applying the image display apparatus 1
to a head-up display 30. For example, the head-up display 30 is
stationarily placed in a predetermined position. When the user 50
moves closer to the head-up display 30, the user 50 can visually
recognize the image displayed on the head-up display 30 as, for
example, a virtual image. The head-up display 30 can be suitably
used, for example, in a driver assist function of a vehicle, a
digital signage, and the like.
The following is a description of an example application to the
head-mounted display 20 as illustrated in FIG. 7A.
FIGS. 8A and 8B are diagrams illustrating the user 50 wearing the
head mounted display 20 when seen from above the head of the user
50. FIG. 8A shows the light guide plate 2a placed parallel to the
user's pupil 51. FIG. 8B shows the light guide plate 2a placed in
an inclined position with respect to the user's pupil 51. Where the
light guide plate 2a is placed in an inclined position with respect
to the user's pupil 51 as illustrated in FIG. 8B, the image
generation unit 3 can be placed closer to an ear 52 of the user 50,
and thus a highly compactable and highly designable head-mounted
display 20 can be provided. It is noted that FIGS. 8A and 8B shows
the image display apparatus 1 being placed for each eye of the user
50, but even when the image display apparatus 1 is placed in front
of only one of the left and right eyes, the same effects are
produced.
FIG. 9 is an enlarged diagram of the light guide plate 2a placed an
inclined position as illustrated in FIG. 8A. In this case, the
light guide plate 2a is placed at an inclination angle .beta. with
respect to the user's pupil 51 (i.e., in the x-axis direction).
Because the field of view of the image light used is offset by the
angle .beta., the image generation unit 3 is structured to be also
inclined by the angle .beta. from the first surface 41 of the
coupling prism 4 to allow light to enter the coupling prism 4.
In this manner, where the image display apparatus 1 is applied to a
head-mounted display, placing the light guide plate 2a in an
inclined position with respect to the user's pupil 51 enables
providing a head-mounted display with a better fit to the head and
with higher designability.
<About Functional Configuration of Head-Mounted Display
20>
FIG. 10 is a block diagram illustrating the functional
configuration of the head mounted display 20 which includes, in
addition to the image display apparatus 1, a control section 21
that controls the overall head-mounted display 20, a sensing
section 23 that detects external information 22, a communication
section 25 that communicates with external equipment 24, a power
supply section 26 that supplies the power, a storage section 27
that storing information, an operation input section 28 and the
like. It is noted that FIG. 10 shows only control lines and
information lines which are considered necessary for description,
and all the control lines and information lines are not necessarily
shown.
The external information 22 includes, for example, the conditions
(position, orientation, motion) of the user 50, the conditions of
outside (brightness, sound, spatial information), and the like.
Examples of the sensing section 23 detecting the conditions
(position, orientation, motion) of the user 50 include a tilt
sensor, an acceleration sensor, a GPS sensor and the like. Examples
of the sensing section 23 detecting the conditions of outside
(brightness, sound, spatial information) include an illuminance
sensor, a sound sensor, an infrared sensor (imaging device).
The communication section 25 is a device communicating with the
external equipment 24 such as information on the internet, a smart
phone, a tablet, PC and the like. For example, Bluetooth
(Registered Trademark), Wifi (Registered Trademark), and the like
may be used for the communication section 25.
The operation input section 28 receives the operation of the user
50 to operate the head-mounted display 20. Specifically, for
example, speech recognition using a sound sensor, touch-panel input
using a pressure-sensitive sensor or a capacitance sensor, gestures
input using an infrared sensor, and the like may be used for the
operation input section 28.
Second Embodiment
A second embodiment includes a modification to the light guide
plate 2a in the first embodiment, and in this configuration, the
partially reflective surface array is divided into a plurality of
regions (hereinafter referred to as "segments" along in the
arrangement direction of the partially reflective surfaces, and the
inter-surface spacing of the partially reflective surfaces is
varied on a segment-by-segment basis.
FIGS. 11A and 11B are diagrams illustrating the configuration of a
light guide plate 2b according to the second embodiment, in which
FIG. 11A is a front view and FIG. 11B is a plan view. The light
guide plate 2b includes the first internal reflective surface 6 and
the second internal reflective surface 7 which are approximately
parallel to each other. The light guide plate 2b has the partially
reflective surface array 10 therein, the partially reflective
surface array 10 including the number N of partially reflective
surfaces 8.sub.1 to 8.sub.N. Here, the partially reflective
surfaces 8 are approximately equal in reflectivity to each
other.
The partially reflective surface array 10 is divided into a
plurality (three in the embodiment) of segments A1 to A3 (indicated
by a dot-and-dash line) along the arrangement direction of the
partially reflective surfaces 8. The partially reflective surfaces
8 within the same segment are approximately equal in inter-surface
spacing L. Between adjacent segments, the partially reflective
surfaces 8 are arranged in such that the inter-surface spacing L in
one segment located closer to the light guide plate end 9 is
smaller than that in the other. Therefore, in comparison with the
light guide plate 2a described in the first embodiment, the number
of types of substrate thickness required for manufacturing is
decreased, and thus a light guide plate can be provided at low
cost.
The placement of the partially reflective surfaces 8 of the light
guide plate 2b is described. When a segment number k is an integer
from 1 to 3, then an inter-surface spacing of the partially
reflective surfaces 8 belonging to the segment Ak is indicated as
L.sub.Ak, and a width is indicated as H.sub.Ak. Also, the number of
partially reflective surfaces 8 belonging to the segment Ak is
assumed as N.sub.Ak. That is, the number N.sub.Ak of partially
reflective surfaces 8 within the segment is variable rather than
constant. After the light with intensity I.sub.0 entering through
the incident surface 5 of the light guide plate 2a passes through
the segment A1, due to the N.sub.A1 partially reflective surfaces
8, the intensity decreases from I.sub.0 to
I.sub.A1=(1-R)).sup.NA1.times.I.sub.0. Therefore, in order to
uniform the image light to be projected, an inter-surface spacing
L.sub.A2 of the partially reflective surfaces 8 in the segment A2
is required to be smaller than the inter-surface spacing L.sub.A1
to increase the luminous flux density. Hence, the relationship is
L.sub.A2<L.sub.A1.
Next, the lower limit of the inter-surface spacing L.sub.A2 is
determined. In the segment A2, for projection of the image light at
intensity equivalent to that in the segment A1, the inter-surface
spacing L.sub.A2 is required only to be decreased by (1-R)).sup.NA1
corresponding to a decrease in light intensity, i.e., to be
L.sub.A2=L.sub.A1(1-R).sup.NA1. If the inter-surface spacing
L.sub.A2 is smaller than L.sub.A1(1-R)).sup.NA1, the number of
partially reflective surface is increase due to a too small
inter-surface spacing, leading to an increase in manufacturing
costs. Hence, the inter-surface spacing L.sub.A2 is within the
following range.
L.sub.A1.times.(1-R).sup.NA1.ltoreq.L.sub.A2<L.sub.A1 (26)
Similarly, the inter-surface spacing L.sub.A3 is within the
following range.
L.sub.A2.times.(1-R).sup.NA2.ltoreq.L.sub.A3<L.sub.A2 (27)
Here, the example of division into three segments A1 to A3 has been
described, but the number of segments may be two or may be more
than three.
For example, the case of the number of segments being M is
described (M is an integer of 2 or greater). It is assumed that a
k-th segment from the incident surface 5 of the light guide plate
2b is Ak, that the number of partially reflective surfaces 8
existing in the segment Ak is N.sub.Ak, and that the inter-surface
spacing is L.sub.Ak (k is an integer from 1 to M). In this case,
for the same reason as the above, the inter-surface spacings
L.sub.Ak, L.sub.Ak+1 of the partially reflective surfaces 8 for
projection of uniform image light is required to satisfy the
following relationship:
L.sub.Ak.times.(1-R).sup.NAk.ltoreq.L.sub.Ak+1<L.sub.Ak(1.ltoreq.k.lto-
req.M-1) (28)
As in the case of the first embodiment, for prevention of a partial
loss of the image, assuming that the thickness of the light guide
plate is T and the angle formed by the partially reflective surface
8 and that the second internal reflective surface 7 is .theta., the
largest inter-surface spacing Lm is set to satisfy the following:
L.sub.A1.gtoreq.T.times.cos .theta. (29) Also, as in the case of
the first embodiment, in terms of costs, the largest inter-surface
spacing L.sub.A1 is set to satisfy the following:
L.sub.A1.gtoreq.T.times.cos .theta./2 (30)
The limit of reflectivity R may be determined as in the case of the
first embodiment, which can be expressed by:
R.ltoreq.6/(EB+2.times.ER.times.tan(FOV/2)) (31)
When the eye relief ER=20 mm and the eye box EB=10 mm, if the limit
of the reflectivity R is determined from expression (31), in the
case of 20 degrees.ltoreq.FOV<30 degrees, the reflectivity R may
be set at 30% or lower; in the case of 30 degrees.ltoreq.FOV<40
degrees, the reflectivity R may be set at 25% or lower; and in the
case of 40 degrees.ltoreq.FOV<50 degrees, the reflectivity R may
be set at 21% or lower.
In the embodiment, the range of available reflectivity is also
extended as compared with the conventional light guide plate. For
example, in the use in 30 degrees.ltoreq.FOV<40 degrees, the
conventional light guide plate requires reflectivity R 4%, whereas
the range of reflectivity R in the light guide plate in the
embodiment is extended to be equal to or less than 25%. As a
result, the light use efficiency of the light guide plate improves
about 2.9-fold as compared with the conventional light guide
plate.
As described above, in the second embodiment, the partially
reflective surface array 10 of the light guide plate 2b is divided
into a plurality of segments along the arrangement direction of the
partially reflective surfaces 8. The partially reflective surfaces
in the same segment are approximately equal in inter-surface
spacing, and the inter-surface spacing of partially reflective
surfaces is different between adjacent segments. And, the closer to
the light guide plate end 9 the segment is located, the smaller the
inter-surface spacing is. As a result, even if high reflectivity is
set for the partially reflective surfaces, the image light of
uniform brightness can be projected, and in turn a light guide
plate capable of projecting image light of uniform brightness with
high light use efficiency can be provided.
Third Embodiment
In a third embodiment, the uniforming element is configured to vary
reflectivity among partially reflective surfaces within the light
guide plate.
FIGS. 12A and 12B are diagrams illustrating the configuration of a
light guide plate 2c according to the third embodiment, in which
FIG. 12A is a front view and FIG. 12B is a plan view. The light
guide plate 2c includes the first internal reflective surface 6 and
the second internal reflective surface 7 which are approximately
parallel to each other. The light guide plate 2c has the partially
reflective surface array 10 therein, the partially reflective
surface array 10 including the number N of partially reflective
surfaces 8. In this configuration, the number N of partially
reflective surfaces 8 are approximately equal in inter-surface
spacing L, and the reflectivity R differs among the partially
reflective surfaces 8.
The configuration of the partially reflective surfaces of the light
guide plate 22c is described. The N partially reflective surfaces 8
are individually denoted as 8.sub.n in order from the incident
surface 5 (n is an integer from 1 to N). It is assumed that
reflectivity of each partially reflective surface 8.sub.n is
R.sub.n and that the inter-surface spacing of the partially
reflective surfaces 8 is L. The partially reflective surfaces 8
increase in reflectivity R from the incident surface 5 toward the
light guide plate end 9 in order to uniform the intensity
distribution of the image light projected by the light guide plate
2.
Next, desirable reflectivity R of the partially reflective surface
8 is described. It is assumed that, when the image light of light
intensity I.sub.0 enters through the light-guide-plate incident
surface 5, light intensity of the light reflected off the partially
reflective surface 8.sub.n is I.sub.n. At this time, I.sub.n can be
expressed by the following: I.sub.n=(1-R.sub.1)(1-R.sub.2) . . .
(1-R.sub.n-1)R.sub.nI.sub.0 (32)
The relationship between reflectivity R.sub.n+1 and reflectivity
R.sub.n of the adjacent partially reflective surfaces is
determined. The amount of light propagating through the light guide
plate decreases as the light is propagated deeper into the light
guide plate by the partially reflective surfaces. Therefore, for
projection of the image light of uniform brightness, the partially
reflective surfaces are required to increase in reflectivity from
the incident surface 5 toward the light guide plate end 9. Hence,
the relationship R.sub.n<R.sub.n+1 is established.
Then, the upper limit of R.sub.n+1 is determined. If the (n+1)-th
light intensity I.sub.n+1 and the n-th light intensity I.sub.n are
equal, uniform image light can be projected. If I.sub.n+1=I.sub.n
is rewritten as the relationship of reflectivity using expression
(32), R.sub.n+1=R.sub.n/(1-R.sub.n) is obtained. Hence, the
reflectivity R is required to be within the range of:
R.sub.n<R.sub.n+1.ltoreq.R.sub.n/(1-R.sub.n)(1.ltoreq.n.ltoreq.N-1)
(33)
In terms of the see-through characteristics, the first R.sub.1 is
desirably set such that the highest reflectivity R.sub.N becomes
30% or lower.
As in the case of the first embodiment, for prevention of a partial
loss of the image, the inter-surface spacing L of the partially
reflective surfaces 8 is desirably set as: L.ltoreq.T.times.cos
.theta. (34) and, in terms of costs, it is desirably set as:
L.gtoreq.T.times.cos .theta./2 (35)
Next, for the most uniform image light to be projected, i.e., for
R.sub.n+1=R.sub.n/(1-R.sub.n), the limit of available reflectivity
is determined. The highest reflectivity in the light guide plate 2c
is reflectivity R.sub.N of the partially reflective surface 8.sub.N
located closest to the light guide plate end 9, and recurrence
formula R.sub.n+1=R.sub.n/(1-R.sub.n) is solved to obtain
R.sub.N=R.sub.1/(1-(N-1)R.sub.1). In order to set reflectivity
R.sub.N at 30% or lower, R.sub.1/(1-(N-1)R.sub.1)<0.3 (36) is
required to be satisfied.
Also, using the length S in the x direction of the region of the
light guide plate in which the partially reflective surfaces are
arranged, the spacing L of the partially reflective surfaces, and
the angle .theta. formed by the partially reflective surface 8 and
the second internal reflective surface 7, the number N of partially
reflective surfaces can be written as N=S.times.sin .theta./L.
Hence, expression (36) is written as:
R.sub.1<0.3/(1+0.3.times.S.times.sin .theta./L) (37)
Using expressions (17) and (34), the above expression (37) can be
expressed as: R.sub.1<0.3/(1+0.3.times.W.times.sin .theta./T)
(38)
As described earlier, T is the thickness of the light guide plate,
the angle .theta. is the angle formed by the partially reflective
surface 8 and the second internal reflective surface 7, and W is
the visibility width. Using eye relief ER and eye box EB, and FOV,
W=EB+2.times.ER.times.tan(FOV/2) can be written.
In order to place the light guide plate in front of the user's eye,
in terms of designability, the thickness T is preferably as thin as
possibly, and thickness T.ltoreq.3 mm is required. Also, when angle
.theta.=25 degrees, if expression (38) is written under the limit
T.ltoreq.3 mm, using eye relief ER and eye box EB and FOV,
R.sub.1<6.4/(EB+2.times.ER.times.tan(FOV/2)+21) (39) is
written.
When the eye relief ER=20 mm, and the eye box EB=10 mm, if a
specific limit of the reflectivity R.sub.1 is determined from
expression (39), in the case of 20 degrees.ltoreq.FOV<30
degrees, reflectivity R.sub.1 may be set at 15% or lower; in the
case of 30 degrees.ltoreq.FOV<40 degrees, the reflectivity
R.sub.1 may be set at 14% or lower; and in the case of 40
degrees.ltoreq.FOV<50 degrees, the reflectivity R.sub.1 may be
set at 13% or lower.
In the embodiment, the range of available reflectivity is also
extended as compared with the conventional light guide plate. For
example, in the use in 30 degrees.ltoreq.FOV<40 degrees, the
conventional light guide plate requires reflectivity R.ltoreq.4%,
whereas in the light guide plate in the embodiment, the
reflectivity R.sub.1 of the partially reflective surface having the
lowest reflectivity is available up to 14%. As a result, the light
use efficiency of the light guide plate improves about 2.8-fold as
compared with the conventional light guide plate.
As described above, in the configuration in the third embodiment,
the partially reflective surfaces 8 within the light guide plate 2c
varies in reflectivity in such a manner as to increase in
reflectivity from the incident surface 5 toward the light guide
plate end 9. As a result, even if high reflectivity is set for the
partially reflective surfaces, the image light of uniform
brightness can be projected, and in turn a light guide plate
capable of projecting image light of uniform brightness with high
light use efficiency can be provided.
Fourth Embodiment
A fourth embodiment includes a modification to the light guide
plate 2c in the third embodiment, and in this configuration, the
partially reflective surface array is divided into a plurality of
segments along in the arrangement direction of the partially
reflective surfaces, and the reflectivity of the partially
reflective surfaces is varied on a segment-by-segment basis.
FIGS. 13A and 13B are diagrams illustrating the configuration of a
light guide plate 2d according to the fourth embodiment, in which
FIG. 13A is a front view and FIG. 13B is a plan view. The light
guide plate 2d includes the first internal reflective surface 6 and
the second internal reflective surface 7 which are approximately
parallel to each other. The light guide plate 2d has the partially
reflective surface array 10 therein, the partially reflective
surface array 10 including the number N of partially reflective
surfaces 8.sub.1 to 8.sub.N. Here, the partially reflective
surfaces 8 are approximately equal in inter-surface spacing L
(width H).
The partially reflective surface array 10 is divided into a
plurality (three in the embodiment) of segments A1 to A3 along the
arrangement direction of the partially reflective surfaces 8. The
partially reflective surfaces 8 within the same segment are
approximately equal in reflectivity R. Between adjacent segments,
the reflectivity R in one segment located closer to the light guide
plate end 9 is set higher than that in the other, in order to
uniform the intensity distribution of the image light to be
projected. In comparison with the light guide plate 2c in the third
embodiment, the number of types of coatings for the partially
reflective surfaces 8 required for manufacturing is decreased, and
thus a light guide plate can be provided at low cost.
Next, a desirable range of reflectivity R of the partially
reflective surfaces 8 is described. When a segment number k is an
integer from 1 to 3, reflectivity of the partially reflective
surfaces 8 belonging to the segment Ak is indicated as R.sub.Ak.
Also, the number of partially reflective surfaces 8 belonging to
the segment Ak is assumed as N.sub.Ak.
After the light with intensity I.sub.0 entering through the
incident surface 5 of the light guide plate 2d passes through the
segment A1, due to the N.sub.A1 partially reflective surfaces 8,
the intensity decreases from I.sub.0 to
I.sub.A1=(1-R.sub.A1).sup.NA1.times.I.sub.0. Therefore, in order to
uniform the image light to be projected, reflectivity R.sub.A2 of
the partially reflective surfaces 8 in the segment A2 is required
to be higher than the reflectivity R.sub.A1 to increase the
luminous flux density. Hence, the relationship is
R.sub.A2>R.sub.A1.
Next, the upper limit of the reflectivity R.sub.A2 is determined.
In the segment A2, for projection of the image light at intensity
equivalent to that in the segment A1, the reflectivity R.sub.A2 is
required only to be increased by the inverse of a decrease in light
intensity, i.e., to be R.sub.A2=R.sub.A1/(1-R.sub.A1).sup.NA1.
Hence, a desire range of the reflectivity R.sub.A2 is:
R.sub.A1<R.sub.A2.ltoreq.R.sub.A1/(1-R.sub.A1).sup.NA1 (40)
Similarly, a desirable range of the reflectivity R.sub.A3 is:
R.sub.A2<R.sub.A3.ltoreq.R.sub.A2/(1-R.sub.A2).sup.NA2 (41)
Here, the example of division into three segments A1 to A3 has been
described, but the number of segments may be two or may be more
than three.
For example, the case of the number of segments being M is
described (M is an integer of 2 or greater). It is assumed that a
k-th segment from the incident surface 5 of the light guide plate
2b is Ak, that the number of partially reflective surfaces 8
existing in the segment Ak is N.sub.Ak, and that the reflectivity
is R.sub.Ak (k is an integer from 1 to M). In this case, for the
same reason as the above, the reflectivity R.sub.Ak, R.sub.Ak+1 of
the partially reflective surfaces 8 for projection of uniform image
light is required to satisfy the following relationship:
R.sub.Ak<R.sub.Ak+1.ltoreq.R.sub.Ak/(1-R.sub.Ak).sup.NAk
(1.ltoreq.k.ltoreq.M-1) (42)
As in the third embodiment, an available range of the reflectivity
R.sub.A1 of the partially reflective surface having the lowest
reflectivity is given by:
R.sub.A1<6.4/(EB+2.times.ER.times.tan(FOV/2)+21) (43)
When the eye relief ER=20 mm, and the eye box EB=10 mm, if a
specific limit of the reflectivity R.sub.1 is determined from
expression (43), in the case of 20 degrees.ltoreq.FOV<30
degrees, the reflectivity R.sub.A1 may be set at 15% or lower; in
the case of 30 degrees.ltoreq.FOV<40 degrees, the reflectivity
RA1 may be set at 14% or lower; and in the case of 40
degrees.ltoreq.FOV<50 degrees, reflectivity R.sub.A1 may be set
at 13% or lower.
In the embodiment, the range of available reflectivity is also
extended as compared with the conventional light guide plate. For
example, in the use in 30 degrees.ltoreq.FOV<40 degrees, the
conventional light guide plate requires reflectivity R.ltoreq.4%,
whereas in the light guide plate in the embodiment, the
reflectivity R.sub.A1 of the partially reflective surface having
the lowest reflectivity is available up to 14%. As a result, the
light use efficiency of the light guide plate improves about
2.8-fold as compared with the conventional light guide plate.
As described above, in the fourth embodiment, the partially
reflective surface array 10 of the light guide plate 2d is divided
into a plurality of segments along the arrangement direction of the
partially reflective surfaces 8. The partially reflective surfaces
in the same segment are approximately equal in reflectivity, and
the reflectivity of the partially reflective surface is different
between adjacent segments. The closer to the light guide plate end
9 the segment is located, the higher the reflectivity is. As a
result, even if high reflectivity is set for the partially
reflective surfaces, the image light of uniform brightness can be
projected, and in turn a light guide plate capable of projecting
image light of uniform brightness with high light use efficiency
can be provided.
Fifth Embodiment
In a fifth embodiment, the uniforming element is configured to
apply a light control coating layer having transmittance
distribution to the outer side of the first internal reflective
surface 6 within the light guide plate.
FIGS. 14A and 14B are diagrams illustrating the configuration of a
light guide plate 2e according to the fifth embodiment, in which
FIG. 14A is a front view and FIG. 14B is a plan view. The light
guide plate 2e includes the first internal reflective surface 6 and
the second internal reflective surface 7 which are approximately
parallel to each other. The light guide plate 2e has the partially
reflective surface array 10 therein, the partially reflective
surface array 10 including the number N of partially reflective
surfaces 8. The number N of partially reflective surfaces 8 are all
equal in inter-surface spacing L, and also are approximately equal
in reflectivity R. Further, a light control coating layer 12 for
uniformization of image brightness is applied to the outer side of
the first internal reflective surface 6 for projection of the image
light to the user. The region in which the light control coating
layer 12 is applied includes the range in which at least image
light is reflected off the partially reflective surfaces 8 to pass
through the first internal reflective surface 6.
The intensity of light to be reflected off the partially reflective
surfaces 8 to exit from the light guide plate 2e decreases as the
light travels from the light guide plate incident surface 5 toward
the light guide plate end 9. Therefore, the transmittance
distribution of the light control coating layer 12 is set such that
the transmittance increases from the light guide plate incident
surface 5 toward the light guide plate end 9 in order to uniform
the brightness of the image to be emitted.
Here, the range for forming the light control coating layer 12 is
only a range D0 to D1 (distance B) in which, as illustrated in FIG.
14B, the image light is reflected off the partially reflective
surfaces 8 to pass through the first internal reflective surface 6.
In order to uniform the intensity I.sub.1 to I.sub.N of the image
light to be projected from the light guide plate 2e, the
transmittance F of the light control coating layer 12 is
distributed as expressed by the following expression (where
0<F<1). Here, the origin of the x axis is assumed as the
start point D0 of the light control coating layer 12.
F(x)=x/B.times.(1-(1-R).sup.(N-1))+(1-R).sup.(N-1) (44)
It is noted that the transmittance F of the light control coating
layer 12 may be varied exponentially in the x-axis direction as
expressed by the following expression:
F(x)=(1-R).sup.((1-x/B).times.(N-1)) (45)
The light control coating layer 12 may be applied to the full face
of the first internal reflective surface 6.
As described above, according to the fifth embodiment, applying the
light control coating layer having the transmittance distribution
to the exit surface of the light guide plate 2e enables the
projection of image light of uniform brightness.
Sixth Embodiment
A sixth embodiment includes a modification to the light guide plate
2e in the fifth embodiment, and in this configuration, the
partially reflective surface array is divided into a plurality of
segments along in the arrangement direction of the partially
reflective surfaces, and the transmittance of the light control
coating is varied on a segment-by-segment basis.
FIGS. 15A and 15B are diagrams illustrating the configuration of a
light guide plate 2f according to the sixth embodiment, in which
FIG. 15A is a front view and FIG. 15B is a plan view. The light
guide plate 2f includes the first internal reflective surface 6 and
the second internal reflective surface 7 which are approximately
parallel to each other. The light guide plate 2f has the partially
reflective surface array 10 therein, the partially reflective
surface array 10 including the number N of partially reflective
surfaces 8.sub.1 to 8.sub.N. Here, the partially reflective
surfaces 8 are approximately equal in reflectivity R and
inter-surface spacing L (width H). And a light control coating
layer 13 is included on the outer side of the first internal
reflective surface 6 to uniform the image brightness.
The partially reflective surface array 10 is divided into a
plurality (three in the embodiment) of segments A1 to A3 along the
arrangement direction of the partially reflective surfaces 8. The
N.sub.Ak partially reflective surfaces 8 (k is an integer from 1 to
3) exist in each segment. The light control coating layer 13 is
also divided corresponding to the segments A1 to A3, and the
transmittance is varied. The transmittance F of the light control
coating layer 13 is equal in the same segment. Between adjacent
segments, the transmittance F in one segment located closer to the
light guide plate end 9 is set higher than that in the other, in
order to uniform the intensity distribution of the image light to
be projected. In comparison with the light control coating layer 12
in the fifth embodiment which has transmittance distribution, the
transmittance of the light control coating layer 13 is constant in
the segment, so that the degree of difficulty in manufacturing can
be decreased to achieve cost reduction.
The transmittance F of the light control coating layer 13 in the
light guide plate 2f is described. The light control coating layer
13 is divided into three segments A1 to A3, and transmittance of
the light control coating layer 13 in the segment Ak is assumed as
F.sub.Ak (k is an integer from 1 to 3). Comparing transmittance
between adjacent segments, the transmittance in one segment located
closer to the light guide plate end 9 is set higher than that in
the other in order to uniform the image light to be projected. That
is, the transmittance F.sub.Ak and the transmittance F.sub.Ak+1 of
the light control coating layer 13 between adjacent segments are
set to have the following relationship: F.sub.Ak<F.sub.Ak+1
(1.ltoreq.k.ltoreq.2) (46)
Here, the example of division into three segments A1 to A3 has been
described, but the number of segments may be two or may be more
than three.
For example, the case where the light control coating layer 13 is
divided into M is described (M is an integer of 2 or greater). It
is assumed that a k-th segment from the incident surface 5 of the
light guide plate 2f is Ak, that the number of partially reflective
surfaces 8 existing in the segment Ak is N.sub.Ak, and that the
reflectivity is R (k is an integer from 1 to M). In this case, for
projection of uniform image light from each segment, for the same
reason as the above, the transmittance F.sub.Ak and the
transmittance F.sub.Ak+1 of the light control coating layer 13
between adjacent segments have a relationship given by:
F.sub.Ak<F.sub.Ak+1 (1.ltoreq.k.ltoreq.M-1) (47)
Further, in order to equalize the light intensity I.sub.Ak and the
light intensity I.sub.A+1 at the segment ends of the respective
adjacent segments, using width H and the number N.sub.Ak of
partially reflective surfaces 8 in place of x position in
expression (44), the relationship may be expressed by:
F.sub.Ak+1-F.sub.Ak=H.times.N.sub.Ak+1/B.times.(1-(1-R).sup.(N-1))
(48)
As described above, according to the sixth embodiment, applying the
light control coating layer to the exit surface of the light guide
plate 2f enables the projection of image light of uniform
brightness. At this time, the transmittance is configured to be
varied for each segment of the partially reflective surface array,
so that a light guide plate capable of being easily manufactured at
reduced costs can be provided.
While some embodiments according to the present invention have been
described, the present invention is limited to the abovementioned
embodiments, and encompasses numerous modifications. For example,
the abovementioned embodiments are described in specific details
for facilitating the understanding of the present invention, and
are not necessarily intended to be limited to including all the
configurations described above. Further, a portion of a
configuration in one embodiment may be substituted by a
configuration in another embodiment. A configuration in one
embodiment may be added to a configuration in another embodiment. A
portion of a configuration in each embodiment may be added to,
removed from or substituted by another configuration.
* * * * *